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UNITED STATES DEPARTMENT OF AGRICULTURE 



1 BULLETIN No. 1008 

«a|>NS^I^ Contribution from the Bureau of Entomology ?' 

S\,^'^>J-U L. O. HOWARD, Chief Jf^^^^t. 




Washington, D. C. PROFESSIONAL PAPER November 15, 1921 

RATE OF MULTIPLICATION OF THE HESSIAN FLY. 

By Wl R. McCoNNELL,' 
Entomological Assistant, Cereal and Forage Insect Investigations. 



Page. 

Introductory and historical .. 1 

Methods of investigation 2 

Average number of ova for the prin- 
cipal broods 4 

Fall generation 4 

Spring generation 8 



CONTENTS. 

Page. 



Avei-age number of ova for the prin- 
cipal broods — Continued. 

Summer generation 5 

Sex ratio 6 

The meaning of these figures 6 

Ooncluslons 8 



INTRODUCTORY AND HISTORICAL. 

In the course of investigations of the Hessian fly {Phytophaga 
destructor Say) and its parasites, at the Cereal and Forage Insect 
Field Station of the Bureau of Entomology at Carlisle, Pa., it be- 
came necessary to have accurate data regarding the normal rate of 
multiplication of this insect. Such information was needed, in the 
first place, as a basis for estimating the efficiency of the various 
species of parasites. In the second place, the writer, having been 
greatly impressed by the rapidity with which an outbreak of serious 
proportions may develop in a short time from comparatively small 
sources, and being aware that such an increase is largely due to the 
reduction in effect of some of the important natural checks to the 
increase of the fly, suspected nevertheless that the potential rate 
of multiplication might be much greater than usually has been sup- 
posed. 

The writer has been unable to find satisfactory data on this subject 
in the literature or among other recorded observations. In fact, it ap- 
pears that little attention has ever been paid to the matter, especially 

> Died June 23, 1920. 
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2 ' BULLETIN 1008, U. S. DEPARTMENT OF AGRICULTURE. 

in recent times. In 1861 Wagner - by confining a female Hessian fly 
"^ith wheat plants secured a total of 83 eggs. He evidently carried 
out other experiments, for he concludes that the number of eggs de- 
posited by a female is more than 80 and less than 100. Koeppen ^ in 
1889 stated that, according to Lindemann, a female may deposit up 
to 200 eggs, but believed these figures too high and gave Haberlandt's 
statement that the female deposits only from 40 to 50 eggs. 

The most careful observations up to the present time seem to be 
those made by Enock.* He confined the female insects in cages with 
barley plants, securing in one case a maximum of 158 eggs. He also 
confined females singly in corked vials and secured in this way a 
maximum of 130 eggs. The number of eggs he secured by these 
methods ranged from TO to 158, with an average for 10 cases of 113 
eggs. He concludes that a single female lays from 100 to 150 eggs. 
Although Lugger ^ dissected a female which contained 238 eggs, the 
usual figures given since the date of the paper by Enock are based 
on Enock's conclusion. This conclusion, however, resulted from an 
insufficient number of experiments, and the methods used in obtaining 
the counts will not give accurate results. The adults of the Hessian 
fly are extremely sensitive to conditions of temperature and moisture. 
The females normally oviposit at comparatively low temperatures and 
when moisture in abundance is present. If these conditions are 
reversed or if conditions are unsatisfactory in some other respect, 
the females will die before depositing all their eggs. A new and 
thorough study of the subject therefore seemed imperative. 

METHODS OF INVESTIGATION. 

For the reasons stated, the method of confining female Hessian 
flies in vials and all other methods involving their confinement in 
cages kept in the laboratory were rejected. Cages kept outdoors 
at the usual times when adults are abroad were also dispensed with, 
because any cage modifies natural conditions, more or less, and be- 
cause it is difficult to secure accurate counts where eggs are scat- 
tered over a number of plants. 

The writer is convinced that under normal conditions each female 
deposits practically all of her eggs. It has been found that at the 
time of eclosion each female contains her normal allotment of eggs 
in a well-developed condition; in fact, it is possible to count all of 
the egg9 by dissecting the pupa stage. It seemed simpler and more 

2 Wagner, Balthasar. Observations on the New Gall-Gnat. Fulda, 1861. Trans- 
lated in U. S. Entomological Commission, Third Report. Appendix II, B, 1883, p. [15]. 

^ Koeppen, F. T. Die Schaedlichen Insekten Russlands. St. Petersburg, 1880. 
Translated in U. S. Entomological Commission, Third Report, Appendix IV, 1883, p. [41]. 

* Knock, F. The Life-History of the Hessian Fly, Cecidomyia Destructor, Say. 
In Trans. Ent. Soc. London, p. 332, 1891. 

» Lugger, O. The Hessian Fly. In Minn. Agr. Exp. Sta. Bui. 64, p. 552, 1899. 



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DOCUMENTS DlVii^lON 

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KATE OF MULTIPLICATION OF HESSIAN FLY. 3 

economical of time, therefore, to dissect the females and count the 
number of ova they contained, and this procedure has been followed 
out beginning with the fall brood of 1918. 

Before the dissection of female flies and the counting of their ova 
had been carried very far, other difficulties appeared. These hinged 
chiefly on the question of what females should be counted. At first 
it seemed simple enough to count the number of ova contained in a 
given number of females and average up the results. This, however, 
proved to be insufficient because of the great variation in the results 
obtained from difi^erent lots of females. The females vary greatly ia 
size, and, correspondingly, in the number of ova they contain; and 
the average number of ova per female varies in different fields. 

In the study to determine some of the reasons for this condition of 
affairs, it became evident that the number of ova contained in a 
female is related to the number of puparia in a tiller. On the aver- 
age, the greater the number of puparia in a tiller the smaller the 
number of eggs the resulting females can lay. Since the average 
number of puparia in infested tillers varies in different fieldsi and in 
different years, the difficulty of making a fair average can be readily 
seen. There also appears to be a relation between the date when 
wheat is sown and the average number of eggs for females of the 
fall brood. Puparia obtained from late-sown wheat developed fe- 
males with a reduced capacity for egg production. When work was 
begun on the spring brood in 1920 it was found that the average 
number of eggs varied markedly for the two principal generations. 
All of these conditions and undoubtedly others have to be dealt with 
in some manner in arriving at a fair estimate. 

In view of these facts, separate counts have been made for the 
principal generations. In the case of the fall brood all of the puparia 
found in a number of plants taken from various fields were caged 
and every female which emerged was dissected. Most puparia were 
taken from fields known to be sown early and which, consequently, 
contained flies of all ages ; or else they were taken from fields where 
the age of the plants was unknown. This avoided the giving of un- 
due weight to fields sown late. In this manner many of the data 
accumulated have been eliminated from the general average for the 
fall brood. 

In the case of the spring brood, all plants Avere obtained from a 
number of fields just before harvest. All their puparia were re- 
moved and caged. In two cages extremely long stubbles, which 
probably contained all the puparia of the original plants, were used. 
In this generation allowance must be made for an enormous mor- 
tality due to parasitism and weather. Usually it is not expected 
that more than from 3 to 10 per cent of the flies will remain alive 
at emergence time in the fall, and, of course, allowance must be made 



4 BULLETIN 1008, U. S. DEPARTMENT OF AGRICULTURE, 

for males which are sure to emerge. Taking- all of these facts into 
consideration, success beyond expectations was achieved in the fall 
of 1919, in making counts for the spring brood of 1920.*^ 

AVERAGE NUMBER OF OVA FOR THE PRINCIPAL BROODS. 

In presenting the results of this investigation, it will be necessary 
to consider first the average number of ova which can be laid by 
females of the principal generations. The data for each of these 
generations will be tabulated separately. After this has been done, 
it will be necessary to take into consideration the proportion of the 
two sexes before a sufficient basis can be had for estimating the rate 
of reproduction of the Hessian fly. 

FALL GENERATION. 

In Table 1 are given the results of counting the number of ova 
contained in 107 females. These flies were obtained from six dif- 
ferent fields, as indicated by their cage numbers. It will be noted 
that the average number of eggs per female varies considerably in 
different fields, even when the number of females counted is approxi- 
mately the same. It is thus easy to see w^hat a difficult matter it is 
to obtain a fair average number of eggs per female. The figures, 
however, are as fair as could be obtained for the year in question 
and in the time available. 

Table 1. — Avcruyc nuinher of ova for females of the fall generation of the 
Hessian fly, Carlisle, Pa., 1919. 



Cage No.— 


Number 
of females 
counted. 


Number 
of ova. 


Minimum 

per 

female. 


Maximimi 

per 

female. 


Range. 


Average 

per 
female. 


1620 


28 
26 
36 
9 
6 
2 


8,014 
6,148 
13, 422 
1,361 
1,099 
658 


74 
33 
189 
107 
134 
314 


383 
365 
464 
195 
302 
344 


309 
332 
275 

88 
168 
30 


286.0 


1716 


236.2 


1753 


372. S 


1755 


151.2 


1779 


183. 2 


1795 


329.0 






Totals 


107 


30, 702 


33 


464 


431 


286.9 







The actual number of ova per female varies from 33 to 464, with 
a range of variation of 431 and an average per female of 286.9. 
It may be added that the smallest female found since this work 
began belonged to this brood, but to none of the foregoing cage 
numbers. This female was dissected by Mr. Hill, who found that 
it contained only 11 ova. The largest female ever found belonged 
also to this brood. This female was in a different series and con- 
tained 474 ova. 



« In this connectian the writer wishes to state that two lots of females of the fall 
brood were dissected by Mr. C. C. Hill ; Mr. P. R. Myers has determined the sex of a 
large proportion of tlie flies as they emerged, and both men have helped in collecting 
and caging the material. All of the work has been done in the laboratory at Carlisle, 
Pa., and most of the material has been collected near by. 



RATE OF MULTIPLICATION OF HESSIAN FLY. 5 

SPRING GENERATION. 

For the spring generation the ova have been counted for 160 
females. The most surprising thing about the results obtained, as 
shown in Table 2, is the general average number of ova per female 
of 232.9. This is an average of 54 eggs per female less than was 
found in the case of the fall brood. 

Table 2. — Averaye number of ova for feinules of the spriny yeneration of the 
Hessian fly; counts made ut Carlisle, Pa., in the fall of 1919. 



Cage No.- 


Number 
of females 
counted. 


Total 
number 
of ova. 


Minimum 

per 

female. 


Maximum 

per 

female. 


Range. 


Average 

per 
female. 


1841 


57 
29 
4 
13 
5 
8 
6 
2 
26 
10 


14,591 
6,990 

780 
3,105 

836 
1,356 
1,048 

483 
5,965 
2,106 


79 
113 

34 
188 

71 
102 
100 
194 
116 
108 


386 
372 
297 
350 
255 
214 
227 
289 
370 
309 


307 
259 
263 
162 
184 
112 
127 
95 
254 
201 


256 


1842 


241 


1869 


195 


1875 


238 8 


1876 


167 2 


1880 


169 5 


1881 


174.7 


1882 


241 5 


1883 


229.4 


1891 


210 6 






Totals 


160 


37,260 


34 


386 


352 


232 9 







While this work has been carried through only one spring genera- 
tion, it is nevertheless quite evident that this generation is con- 
stantly lower than the fall generation in its capacity for egg produc- 
tion. The actual minimum number of ova per female is 34, which 
is one more than for the fall brood. The maximum, 386, on the 
contrary, is much lower, and the range of variation is only 352. 

SUMMER GENERATION. 

While data for the flies developing in volunteer wheat are scant, 
the results obtained seem interesting enough to be included as 
Table 3. Only 15 cases are available. In spite of the low average 
number of eggs, this brood in all probability resembles rather closely 
the fall generation in the number of eggs it can lay. If this should 
prove true, it might be that the stage of growth of the plant has an 
important relation to the size of the flies developing in a plant, 
and consequently to the number of ova developing. 

Table ^.—Ai-'eraye number of ova for females of the yenerntlon of the Hessian 
fly develojnny in volunteer tvheat; counts made at Carlisle, Pa., in the sum- 
mer of 1919. 



Cage No. 


Number 
of females 
counted. 


Total 
number 
of ova. 


Minimum 

per 

female. 


Maximum 

per 

female. 


Range. 


Average 

per 
female. 


1990. 


2 
13 


524 
3,027 


177 

54 


347 
448 


170 
394 


262.0 


2090 


232.8 






Totals 


15 


3,551 


54 


448 


394 


236.7 







6 



BULLETIN 1008, U. S. DEPARTMENT OF AGRICULTURE. 



SEX RATIO. 

Before proceeding further with the discussion of the rat^of multi- 
plication, it is necessary to take up the question of the proportion 
existing between the two sexes. In order to acquire data on this 
subject, a beginning was made, more than two years ago, to. record 
the sex of all Hessian flies that emerged in the cages. These cages 
contained onl}'^ puparia separated from their surrounding plant tissue 
and reared in small vials. The puparia had been obtained in various 
localities in the northeastern portion of the United States. In this 
way the sex has been recorded of practically every fly that has 
emerged from a few thousand puparia each year. At the end of 
the first year it appeared that the percentage of females in the 
spring generation was remarkably high. It was not anticipated that 
this excess of females in the spring generation would prove to be con- 
stant, but Table 4 shows that for the second year the percentage of 
females dropped only slightly. Although the rearing of flies from 
the 1919 material has not been completed, a much larger number 
than usual have already been obtained, and the percentage of 
females for 1919 is almost the same as for the two preceding years. 

Table 4. — The sex ratio in the various Hessian fly generations, 1917, 1918, and 

1919. 



Generation. 


Year. 


Number of 
males. 


Number of 
females. 


Total 
reared. 


Per cent of 
females. 


Spring 


f 1917 
i 1918 
I 1919 
/ 1917 
\ 1918 
f 1917 
\ 1918 


168 
146 
623 
749 
1,493 
246 
51 


276 

235 
1,031 

761 
1,313 

225 
45 


444 

381 

1,654 

1,.510 

2,806 

471 

96 


62.2 
61.7 


Fall 


62.3 
50.4 


Volunteer wheat 


46.8 
47.8 




46.9 



For the three years the average percentage of females is 62.1, and 
it seems quite safe to assume now that at least 60 per cent of the 
spring brood develop into females. In the case of the fall genera- 
tion the sexes appear to be approximately equal in numbers, although 
figures for 1918 are rather low in females. In volunteer wheat there 
are a very small number of cases from 1918, but it appears in gen- 
eral that the sexes are about equal in the partial broods developed in 
the summer. 

It is noAv evident enough that the spring brood is the one which 
shows the greatest variation from normal, both in the high percent- 
age of females produced and in the reduced number of eggs these 
females can lay. Both may be related phenomena with a common 
explanation, but this expla^iation will have to be left to the imagi- 
nation at the present time. 

THE MEANING OF THESE FIGURES. 

In order to demonstrate fully the meaning of the figures which 
have been submitted, two examples will be of considerable help. 



KATE OF MULTIPLICATION OF HESSIAN FLY. 7 

Ooing on the assumption that under normal conditions practically 
every egg is laid, let it be assumed that none of the fly stages are 
destroyed by enemies. For the sake of round numbers let 285 be 
taken as the average number of eggs laid by the fall generation and 
230 as the average by the spring generation. It may also be taken 
for granted that one male will fertilize several females, since Enock ^ 
proved by careful methods that one male successfully fertilized as 
many as six females. This will take care of the preponderance of 
females in the spring brood. 

Starting now with a single female of the spring brood emerging 
in the fall, it is found that she can lay 230 eggs. One-half of these 
eggs in the spring will produce females, each of which may lay 285 
eggs. This will give 32,775 as the product of one female working 
through two generations in one wheat crop, w^ith no allowance made 
for multiplication in volunteer wheat. The flies which develop in 
volunteer wheat in the East are usually comparatively small in num- 
ber and are heavily parasitized. If it be assumed that all these flies 
remain alive in the stubbles until the next fall, there will be a total 
of 60 per cent of the flies emerging as females, or 19,665. If these 
all lay 230 eggs, there will be the enormous total of 4.522,950 flies 
developing in the fall wheat of the second year. The resulting fe- 
males would number 2,261,475 and would give rise in the second 
spring to 644,520,375 fly stages. This result has been obtained in 
two crop years, not allowing for any multiplication in volunteer 
wheat. 

The question may be looked at from another point of view. Prob- 
ably infestation of stubbles is never as low as one puparium per 
acre, although the foregoing computation began with one fly. In 
■one field studied quite carefully during the summer and fall of 1919 
the stubbles were found to be infested at the rate of 4,900,000 per 
acre. A careful examination of the puparia in these stubbles just 
before the emergence season showed that only 4.4 per cent contained 
living fly larvae, parasites having destroyed practically all of the 
remainder. This 4.4 per cent amounts to 215,600 flies per acre. If 
60 per cent of these were females there would be 129,360 females per 
acre ovipositing in the next crop of wheat and producing in it the 
next fall 29,752,800 ova. With similar calculations from a few 
other fields as a basis, a big outbreak in wheat sown in the fall of 
1919 w^s looked for. As a matter of fact, however, it was found 
after the emergence season was over that many of the expected flies 
had failed to emerge from the stubbles. A careful count made in 
the field mentioned above showed that only about one-third of the 
healthy fly larvae had transformed. This amounted to about 1.4 
per cent of the total number of flies. 

^ Op. cit. 



8 BULLETIN 1008, U. S. DEPAETMEXT OF AGRICULTURE. 

Correcting the figures to correspond with Avhat actually happened 
in the field and calculating in the same way as before, it is f©und that 
this extremel}'' small percentage of flies transforming in this stubble 
field in the fall would amount to 68,600 flies per acre, with 60 per cent 
or 41,160 females. These females would lay enough eggs to develop 
into 9,466,800 flies per acre in young wheat sown in the fall of 1919. 
This is a much smaller number than had been anticipated, but it will 
be readily seen that even this is an enormous infestation. If the 
farmer sows about the same acreage from year to year and no flies 
are lost by migration, then the flies from an acre of stubbles would 
proceed to an acre of new wheat. It is calculated that 1,000,000 
plants per acre is a good stand, and in that case this farmer would 
have over nine flies developing on every wheat plant. It will be 
readily seen from this that his chances of securing a wheat crop 
under the conditions named are practically zero. It is also evident 
that anything which helps to cut down this rate of reproduction is 
of enormous benefit. The principal checks to the multiplication of 
the Hessian fly are its parasitic enemies and unfavorable weather 
conditions, and if it were not for them it probably would be im- 
possible inside of two years to grow wheat at all. This illustration, 
it is hoped, will also help to explain how an outbreak may seem to 
develop suddenly from a very low infestation of stubbles. 

CONCLUSIONS. 

The principal points made in this bulletin may be summarized as 
follows : 

1. The rate of multiplication of the Hessian fly is much higher 
than has been realized. 

2. The rate of multiplication is quite different in the two principal 
broods of the Hessian fly, the spring brood laying on an average 
only about 230 eggs per female, whereas the fall brood lays about 285 
eggs per female. 

3. The capacity for reproduction also varies with a number of 
other factors, such as date of sowing, number of puparia per tiller, 
etc. 

4. Because of these various influences, the actual rate of multipli- 
cation will be found to vary from year to year and even from field to 
field, and in years of light infestation the figures will prove too low. 

0. The proportion of males to females varies in the two principal 
generations. In the spring generation about 60 per cent of the flies 
are females ; in the fall generation the sexes are approximately equal 
in number. 

6. By applying these figures, however unsatisfactory the basis may 
be, it is believed that entomologists will be better able to appreciate 
how a Hessian fly outbreak may develop very suddenly, and to pre- 
dict in a more accurate manner the approach of a dangerous outbreak. 

WASHINGTON : GOVERNMENT PRINTING OFFICE : 1921 



LIBRARY OF CONGRESS 



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